1. Field of the Invention
This invention relates to an inscribed meshing planetary gear construction which is preferably applied to a speed increasing or a reduction gear, more particularly, a small-sized speed increasing gear or a reduction gear in which a high output torque is required.
2. Description of the Prior Art
In the prior art, it is widely known to provide a speed increasing gear or a reduction gear employing an inscribed meshing planetary gear construction comprising a first shaft, an external-tooth gear mounted (assembled) on the first shaft through a eccentric body in a state where the external-tooth gear can be rotated eccentric around the first shaft, an internal-tooth gear with which the external-tooth gear is inscribed and meshed, and a second shaft connected to the external-tooth gear through means for transmitting only the rotation component of the external-tooth gear.
An example of the prior art of this construction is shown in FIGS. 6 and 7. This prior art is constructed such that said first shaft is applied as an input shaft, said second shaft is applied as an output shaft and at the same time said construction is applied to a reduction gear by fixing the internal-tooth gear.
Eccentric bodies 3a, 3b are mounted on the input shaft 1 with a predetermined phase difference (180.degree. in this example). The eccentric bodies 3a, 3b are integrated into one body. Two external-tooth gears 5a, 5b are placed on to each of these eccentric bodies 3a, 3b through eccentric bearings 4a, 4b. A plurality of inner roller holes 6 are provided in the external-tooth gears 5a, 5b and then an inner pin 7 and an inner roller 8 are fitted in the roller holes.
External teeth 9 such as trochoidal teeth or circular teeth etc. are at outer circumferences of said external-tooth gears 5a, 5b. The outer teeth 9 are inscribed and meshed with the internal-tooth gear 10 fixed to a casing 12. The internal teeth of the internal-tooth gear 10 are constructed such that an outer pin 11 is loosely fitted to an inner pin hole 13 and held to be easily rotatable.
An inner pin 7 passing through said external-tooth gears 5a, 5b is tightly fitted to or fixed to a flange part 14 of the output shaft 2.
When the input shaft 1 is rotated once, the eccentric bodies 3a, 3b in conjunction rotate once. The external-tooth gears 5a, 5b are apt to oscillatebly rotate around an eccentric axis about shaft 1 through this one revolution of the eccentric bodies 3a and 3b. However, since the rotation is restricted by the internal-tooth gear 10, the external-tooth gears 5a, 5b almost perform an eccentric rotation while being inscribed with the internal-tooth gear 10.
Now, it is assumed that the number of teeth of the external-tooth gears 5a, 5b is N and the number of teeth of the internal-tooth gear 10 is N+1, then the difference between the numbers of teeth is 1. Consequently, the external-tooth gears 5a, 5b are displaced by one tooth relative to the internal-tooth gear 10 fixed to the casing 12 every time the input shaft 1 is rotated. This means that one revolution of the input shaft 1 is decelerated to a revolution of -1/N (- sign show change in direction) of the internal-tooth gear.
Oscillation component of the external-tooth gears 5a, 5b is absorbed by clearances between the inner roller holes 6 and the inner pins 7 and then only the revolution component is transmitted to the output shaft 2 through the inner pins 7.
In this case, the inner roller holes 6a, 6b and the inner pins 7 (inner rollers 8) form an "isokinetic inscribed meshing mechanism".
As a result, finally, a reduction of reduction ratio -1/N can be accomplished.
In the example of this prior art, the internal-tooth gear of the inscribed meshing planetary gear construction is fixed, the first shaft is an input shaft and the second shaft is an output shaft. However, a reduction gear can be also constructed by fixing the second shaft and applying the first shaft as an input shaft and the internal-tooth gear as an output shaft. Furthermore, a speed increasing gear can also be constructed by reversing these inputs and outputs.
By the way, in the above-described prior art, two external-tooth gears are provided mainly because of consideration of increase in transmission capacity, increase of strength or maintenance of rotational dynamic balance. Generally, external teeth of these two external-tooth gears are simultaneously cut (manufactured or processed) in a condition where they are superimposed in order to reduce the number of steps of manufacturing. In this case, these two external-tooth gears are assembled so that the external tooth of one of the external-tooth gears and the external tooth of the other external-tooth gear(s), which had been cut at same position during manufacturing respectively, are simultaneously meshed with the internal-tooth gear during operation in order to cancel manufacturing errors with each other.
For example, in case of "two" external-tooth gears, phases in the maximum eccentric direction are deviated by 180.degree. from each other. Therefore, the two external-tooth gears are rotated by 180.degree. and assembled so that said external teeth cut at the same position are simultaneously meshed with the internal-tooth gear during operation. Under an idea similar to the above, in case of "three" external-tooth gears, since the respective maximum eccentric directions are rotated by 120.degree. from one another, the external-tooth gears are rotated by 120.degree. and assembled so that said external teeth cut at the same position are simultaneously meshed with the internal-tooth gear during operation.
This is because of the idea that said external teeth cut at the same position are simultaneously meshed with the internal-tooth gear whereby manufacturing errors are cancelled from each other.
Recently, also in the field of the speed increasing and reduction gear employing the inscribed meshing planetary gear construction of this kind, miniaturization and higher accuracy have been strongly demanded.
Actually, to respond to such a demand by improving the working accuracy is one aspect and the limitation of cost has to be considered.